Dosing device

ABSTRACT

The invention relates to a dosing device for mixing additive into a basic flow, comprising: a passage for the basic flow; an inlet for the additive in the passage; a container for the additive connected to the inlet via a dosing mechanism and having a controllable drive for discharge of the additive into the inlet; weighing means for determining the weight of at least the container with the additive; and a control for the drive which is connected to the weighting means for setting a dosed discharge, wherein the dosing mechanism is arranged on the inlet via the weighing means.

[0001] The present invention relates to a dosing device for adding anadditive to a basic flow. Such dosing devices are generally known,whereby for instance dye can be added to a raw material for an extrusionprocess so as to provide the finished product with colour.

[0002] In such known dosing devices use is usually made of a rigidconstruction to arrange a container for the additive. Weighing means canthen be placed between the rigid construction and the container for theadditive.

[0003] A drawback of the known art is that such constructions areusually unnecessarily complex, particularly when a narrowing occurs inthe passage for the basic flow with raw material in the flow directionprior to a follow-on operation. It is desirable here to keep as short aspossible the distance over which additive for dosing must be displacedby a dosing mechanism. This is desirable in order to obtain the quickestpossible response to a change in the operation of the dosing mechanism,for instance if a drive co-acting with the dosing mechanism iscontrolled to a higher or lower dosing rate by a control connected tothe weighing means.

[0004] The present invention has for its object to obviate at least theabove stated and possibly also other problems of the known dosingdevices, for which purpose a dosing device is provided which isdistinguished according to the invention by the combination of measuresaccording to the main claim.

[0005] According to the present invention a dosing device has a simpleand elegant construction with which precise dosing is possible.

[0006] The weighing means preferably comprise a compensation forshifting the centre of gravity of the dosing device with the additivetherein. Such a preferred embodiment is particularly favourable inrespect of the above described narrowing in the passage for the basicflow, wherein the container can have a form corresponding with thisnarrowing. This, once again, to keep as short as possible the distancebetween the container for the additive and the passage for the basicflow, and hereby also the dosing mechanism. As the level of additive inthe container falls, a shifting of the centre of gravity herein takesplace which, in a configuration wherein the dosing mechanism isconnected to the inlet via the weighing means, influences the weightweighed using the weighing means. This is particularly the case when thecontainer is not wholly supported by the weighing means, but theweighing means actually form a connection between the inlet and thedosing mechanism. By providing the weighing means with such acompensation a reliable measurement of the weight of at least thecontainer and the additive therein is obtained at all times, as well asa determination of the outflow at a set operation of the dosingmechanism, so that accurate dosing is possible, irrespective of anydisplacement or shifting of the centre of gravity of the container withthe additive in a lateral direction.

[0007] The weighing means can be designed in different ways, forinstance as a load cell, which is then for instance an off-centre loadcell, which is a type of load cell which comprises a compensation forthe distance to the weight load—within limits known in advance such asthe size of a weighing platform extending from the load cell forcarrying the weight load for weighing.

[0008] A dosing device according to the invention preferably has thefeature that the inlet comprises a neck part, through which the passageextends, and an opening forming the inlet for loosely receiving thereinan outlet of the dosing mechanism. Thus is ensured that contact betweenthe dosing mechanism and the opening cannot have any adverse effect onmeasurement by the weighing means, since such contact is avoided.

[0009] In a further preferred embodiment a dosing device according tothe invention has the feature that the dosing mechanism comprises a wormscrew in a cylinder which is connected to the drive and forms a dosingcylinder, wherein the worm screw and the cylinder form a unit.Surprisingly, such a dosing cylinder is found to have a very readilypredictable dosing characteristic, although in many cases it displays anon-linear relation between the rotation speed of the dosing cylinderand the discharge thereof in mass per unit of time. When these are knownin advance, such readily predictable characteristics are very useful foradjusting the control serving to manage the drive on the basis ofmeasurement data from the weighing means. Such readily predictablecharacteristics, which are co-dependent on the material properties ofthe additive used at a particular moment and which can be determined byexperiment, can be used to obtain a desired discharge of additive intothe basic flow. Very slight fluctuations, for instance in the materialproperties, will already affect the discharge if this is setautomatically on the basis of a predetermined characteristic. In afurther preferred embodiment a control is thus provided which is adaptedto bring about with a corrective algorithm an adjustable dosed dischargeby adjusting the rotation speed of the dosing cylinder caused by thedrive on the basis of determinations of weight made by the weighingmeans. The characteristics of the dosing cylinder known in advance canherein be used on the basis of the material properties of the additivefor dosing in order to use a first good approximation of the desiredrotation speed to obtain a dosed discharge, whereafter the correctivealgorithm is applied to obtain a precise adjustment of the dosed desireddischarge.

[0010] The invention will be further described hereinbelow withreference to the annexed figures, wherein:

[0011]FIG. 1 shows a partly cut-away perspective view of a dosing deviceaccording to the present invention; and

[0012]FIG. 2 shows a characteristic associated with the dosing mechanismshown in FIG. 1 for a determined additive.

[0013] The dosing device 1 shown in FIG. 1 comprises a container 2 foradditive 3, which can be introduced into container 2 using a filler cap4. Additive 3 has to be added to a basic flow 5, which flows through apassage 6. A mixture of the basic flow 5 and the additive 3 addedthereto is carried in the direction of arrow A to a further processingdevice 7, for instance an injection-moulding machine or an extruder.

[0014] Passage 6 debouches into a neck part 8 with an inlet 9 for infeedof additive 3. Placed into inlet 9 is a dosing mechanism 10 whichcomprises a dosing cylinder 11 for displacing additive 3 from container2 to neck part 8.

[0015] Dosing cylinder 11 is connected to a motor 12, for instance astepping motor, which is driven by a control 13, which is connected inturn to a load cell 14 of the single point type which forms weighingmeans, and which comprises a compensation for shifting the centre ofgravity of the container with the additive 3 therein. A signalrepresentative of the weighed weight of container 2 with additive 3 issent to control 13 by load cell 14, which can be an off-centre loadcell. Control 13 processes the data coming from load cell 14 and drivesthe motor 12 on the basis hereof, whereby a higher or lower rotationspeed of dosing cylinder 11 can be realized as desired. The motor thusforms a drive, controllable by the control, for the dosing mechanismformed by dosing cylinder 11.

[0016] Load cell 14 forms the connection between neck part 8 andcontainer 2. Dosing cylinder 11 protrudes loosely into the inlet 9 inneck part 8. Owing to the clearance between dosing cylinder 11 and inlet9 of neck part 8 a reliable measurement can be obtained using load cell14 in respect of the weight of container 2 and additive 3 therein. Theclearance is however so small that no loss of material from the basicflow or additive can occur therealong.

[0017] Container 2 has a form corresponding with the narrowed form ofpassage 6. In this manner the dosing cylinder 11 can be kept short,together with the distance between the container and the neck part. Achange in the rotation speed of dosing cylinder 11 achieved with motor12 thus has practically immediate result, and the configuration iscompact and simple.

[0018] Owing to the form of container 2 corresponding with the narrowingin passage 6 the centre of gravity of container 2 with the additive 3therein shifts in obliquely downward direction as more additive 3 isintroduced into the basic flow 5 with the dosing mechanism. Such alateral displacement of the centre of gravity of container 2 withadditive 3 results in a changing moment of force, which would influencethe measurement with load cell 14. According to the present inventionhowever, load cell 14 is provided with a compensation for such a shift.

[0019] As already noted above, such a load cell is for instance anoff-centre load cell. Other load cells compensated for such shifts incentres of gravity, or other weighing means which are not sensitive tosuch shifts, can also be applied. On neck part 8 is for instance mounteda platform on which the container rests in freestanding manner with moreconventional scales between the platform and container 2.

[0020] The stated off-centre load cell is active within a predeterminedrange, which for instance forms a platform with known dimensions. Such aplatform is for instance 400 mm×400 mm. Such a platform can bephysically present or, as in the case of FIG. 1, formed by the bottom ofcontainer 2. The said off-centre load cell can weigh very accuratelywith a total error in the order of magnitude of 0.01% or even less. Avery precise dosing is thus possible herewith.

[0021] Dosing cylinder 11 comprises a helical worm 15 which is connectedto a drive shaft 17, which is connected in turn to motor 12. Round thehelical worm 15 is arranged a cylinder which is arranged fixedly onhelical worm 15 and forms a unit therewith. Cylinder 16 thus co-rotateswith helical worm 15 under the driving action of motor 12.

[0022] In this construction the characteristic of dosing mechanism 10displays a non-linearity in the relation between the discharge in massper unit of time and the rotation speed of motor 12 imposed on dosingmechanism 10. Such a non-linear characteristic is plotted in FIG. 2 fora specific type of additive 3. The characteristic shown in FIG. 2 willhave a different progression for a different additive 3 with othermaterial properties. The non-linearity of this characteristic is relatedto the configuration of dosing cylinder 11, wherein cylinder 16 andhelical worm 15 are fixed to each other.

[0023] At a desired discharge Y, two possibilities of which are given inFIG. 2, this can be fed into control 13 in order to drive the motor 12,for instance a stepping motor, on the basis thereof. On the basis ofprior knowledge and perhaps a characteristic of a specific additive 3,an estimate X is made of a desired rotation speed. In the case of thehigher of the two values Y in FIG. 2 an error of estimation is howevermade, for instance due to variation in the material properties pershipment obtained from a producer. At the rotation speed X anticipatedto be associated with value Y a higher discharge than Y is found toresult. On the basis of measurement results of the weight of container 2with additive 3 therein, the control 13 thus provides adjustment of therotation speed by driving motor 12, wherein the rotation speed isreduced to value X′, wherein the desired value of discharge Y isrealized. A similar situation occurs at the lower of the two values Y inFIG. 2, wherein the rotation speed must however be increased from X toX′ in order to obtain the desired value of discharge Y. At the rotationspeed X anticipated on the basis of prior knowledge, a discharge lowerthan the desired discharge Y was obtained.

[0024] It will be apparent that many alternative and additionalembodiments are possible within the scope of the present invention, thescope of protection of which is defined in the appended claims. It isfor instance possible, as alternative to the use of load cells, to alsouse other constructions and still realize a compact configuration withthe shortest possible distance between the container and the neck partand, in the same context, the shortest possible length of the dosingmechanism. As addition, filling means can be provided which, when a lowlevel has been reached in the container, automatically carry outrefilling of the container with additive. If the control operates inintegrating manner, such refilling operations will have to be taken intoaccount because of the discontinuity in the measuring results associatedtherewith. For the skilled person in this technical field it will bepossible to take such measures without difficulty. In contrast to theconfiguration shown in FIG. 1, the load cell can also be arranged on theside of the neck part and the side of the container. It is furtherpossible for more than one load cell to be applied, and for instance forthe combined measuring result of the load cells to be applied. A dampingcan further be arranged in order to suppress or damp vibrations whichcould disturb the measurements. Such a damping can be designed as abellows-shaped element under the load cell, in which for instancedamping fluid, such as oil, water and so on, can then be arranged. It isherein not desirable for the damping to “carry” the load cell such thatthe measuring result is reduced, although in the application accordingto the invention the weight decrease is of particular importance, sincespeed of measurement is also an important factor in a desired operationof the dosing device. The damping can be arranged close to the neckpart, but is expected to be more effective if it is placed close to thecontainer under the load cell. Other variants and alternatives are alsopossible according to the invention as this is not only disclosed in theforegoing but is also defined in the appended claims.

1. Dosing device for mixing additive into a basic flow, comprising: a passage for the basic flow; an inlet for the additive in the passage; a container for the additive connected to the inlet via a dosing mechanism and having a controllable drive for discharge of the additive into the inlet; weighing means for determining the weight of at least the container with the additive; and a control for the drive which is connected to the weighing means for setting a dosed discharge, wherein the dosing mechanism is arranged on the inlet via the weighing means.
 2. Device as claimed in claim 1, wherein the weighing means comprise a compensation for shifting the centre of gravity of the container with the additive therein.
 3. Device as claimed in claim 1 or 2, wherein the weighing means comprise a load cell.
 4. Device as claimed in claims 2 and 3, wherein the load cell is of the off-centre type.
 5. Device as claimed in one or more than one of the foregoing claims, wherein the inlet comprises a neck part, through which the passage extends, and with an opening forming the inlet for loosely receiving therein an outlet of the dosing mechanism.
 6. Device as claimed in one or more than one of the foregoing claims, wherein the dosing mechanism comprises a worm screw in a cylinder which is connected to the drive and forms a dosing cylinder, wherein the worm screw and the cylinder form a unit.
 7. Device as claimed in claim 6, wherein the characteristic of the discharge in mass per unit of time relative to the rotation speed of the dosing cylinder is non-linear and the control is adapted to bring about with a corrective algorithm an adjustable dosed discharge by adjusting the rotation speed of the dosing cylinder caused by the drive on the basis of determinations of weight made by the weighing means. 